Chemical Digestion

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•  Enzymes digest most macromolecules in food into monomers in the small intestine

Food can be digested by a combination of two methods – mechanical digestion and chemical digestion

  • In chemical digestion, food is broken down by the action of chemical agents (such as enzymes, acids and bile)

Stomach Acids

  • The stomach contains gastric glands which release digestive acids to create a low pH environment (pH ~2)
  • The acidic environment functions to denature proteins and other macromolecules, aiding in their overall digestion
  • The stomach epithelium contains a mucous membrane which prevents the acids from damaging the gastric lining
  • The pancreas releases alkaline compounds (e.g. bicarbonate ions), which neutralise the acids as they enter the intestine 


  • The liver produces a fluid called bile which is stored and concentrated within the gall bladder prior to release into the intestine
  • Bile contains bile salts which interact with fat globules and divide them into smaller droplets (emulsification)
  • The emulsification of fats increases the total surface area available for enzyme activity (lipase)


  • Enzymes are biological catalysts which speed up the rate of a chemical reaction (i.e. digestion) by lowering activation energy
  • Enzymes allow digestive processes to therefore occur at body temperatures and at sufficient speeds for survival requirements
  • Enzymes are specific for a substrate and so can allow digestion of certain molecules to occur independently in distinct locations

Examples of Digestive Enzymes

digestive enzymes

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•  The pancreas secretes enzymes into the lumen of the small intestine

Digestive enzymes are secreted predominantly by the pancreas, although other organs also contribute (salivary gland, stomach)

  • The type of enzyme secreted and location of secretion depends on the specific macromolecule required for hydrolysis


  • Carbohydrate digestion begins in the mouth with the release of amylase from the salivary glands (amylase = starch digestion)
  • Amylase is also secreted by the pancreas in order to continue carbohydrate digestion within the small intestine
  • Enzymes for disaccharide hydrolysis are often immobilised on the epithelial lining of the small intestine, near channel proteins
  • Humans do not possess an enzyme capable of digesting cellulose (cellulase) and hence it passes through the body undigested


  • Protein digestion begins in the stomach with the release of proteases that function optimally in an acidic pH (e.g. pepsin = pH 2)
  • Smaller polypeptide chains enter the small intestine where they are broken down by endopeptidases released by the pancreas
  • These endopeptidases work optimally in neutral environments (pH ~ 7) as the pancreas neutralises the acids in the intestine


  • Lipid breakdown occurs in the intestines, beginning with emulsification of fat globules by bile released from the gall bladder
  • The smaller fat droplets are then digested by lipases released from the pancreas

Nucleic Acids

  • The pancreas also releases nucleases which digest nucleic acids (DNA, RNA) into smaller nucleosides

Locations of Enzymatic Digestion

pancreatic enzymes